What is it about a baby’s birthday that marks the transition from formula to cow’s milk? The question of when and why we stop using formulas relates to when babies become dependent upon solids. Both breast milk and formula bring babies to the point of weaning where they are able to support themselves on a variety of age-appropriate foods. This may happen late during a baby’s first year or early into the second year of life.
During the first year we pay close attention to the mineral and protein concentration in a baby’s diet. This is why we mandate that babies be fed only breast milk or formula. The first birthday marks the point where a baby’s body is able to handle many of the foods that adults eat. The additional protein, potassium and sodium found in cow’s milk, for example, is handled by a baby’s kidneys and intestinal tract without a problem. Another reason cow's milk is inappropriate under a year is its lack of appropriate levels of iron, linoleic acid, vitamin E.
But a baby could conceivably lag into her second year feeding Enfamil, Similac or Good Start without any repercussions. It is assumed, however, that toddlers at this point are receiving protein and vitamins from solid food sources. Infant formulas alone are unlikely to provide everything that a toddler needs for growth and development.
For many toddlers, milk may be considered a supplement, a nearly irrelevant source of bonus calories. For those who struggle with solids, however, milk is a key supplement. But as pediatricians we assume that for most kids the buffer offered by milk is a good thing. In fact, whole milk is recommended over lower fat milks to cover for the calories that some picky toddlers lack.
So while the 12-month point for eliminating formula may seem fairly arbitrary, it’s safe to say that it arises from the timing of what babies do on their own (wean) as well as the development of their bodies to take things beyond breast milk or formula. But like so many other things in parenting and pediatrics, we’ve got more wiggle room than you know.